1. Field of the Invention
The present invention relates to facilitated transport membranes for olefin/paraffin separation, and more particularly to silver-containing polymer membranes with improved stability for olefin/paraffin separation and a method for preparing the same.
2. Description of the Prior Art
Olefins, basic raw materials for the chemical industry, are mainly obtained by cracking naphtha or natural gas as mixtures with saturated hydrocarbons with close volatilities, and thus olefins must be separated before they can be used.
Currently, separation of olefin/paraffin mixture is mainly carried out by highly energy-intensive low temperature distillation. As a result, a number of alternatives have been investigated including adsorption and membrane separation.
There was reported a method for separating olefins from paraffins by adsorption using silver or copper salts which reversibly react with olefins (see, Ind. Eng. Chem. Res. 2571 (1998)). However, this separation method using adsorption is disadvantageous because desorption process must be carried out after adsorption, thereby requiring a large equipment and space.
Recently, separation by facilitated transport membranes in which silver salts are used as carriers has attracted much interest because of the low energy consumption, compact apparatus, and simple operation. The basis for the separation is the ability of silver ions to interact reversibly with olefins by forming silver-olefin complexes. There have been many reports on the facilitated transport of olefins by using various membranes such as supported liquid membranes and ion-exchange membranes; however those membranes, as disclosed in U.S. Pat. No. 3,758,603, exhibit high olefin/paraffin selectivity only in the presence of water, requiring costly and undesirable humidification and dehumidification steps in the practical applications.
A solution to this problem is the development of facilitated transport polymer membranes. U.S. Pat. No. 5,670,051 discloses a facilitated transport polymer membrane in which silver salt present in polymer matrix interacts with olefins. According to this patent, separation of olefins from paraffins is conducted in the absence of water, so that loss of the silver salt caused by introduction of gas does not occur. Furthermore, since the polymer membrane can be produced so as to have a reduced thickness of several μm or below, gas permeability can be significantly increased. In addition, much larger amounts of silver salts can be used in comparison with other types of membranes, and thus the separation efficiency can be significantly increased.
However, polymer membranes containing silver salts exhibit some disadvantages that prevent the commercialization of this separation process. One major drawback observed in the polymer membrane is the decrease in membrane performance with time, possibly due to the reduction of silver ions to silver particles by light or impurities.